How Much Did the Moon Heat Young Earth?

Tidal heating may have raised the surface temperature of early Earth and triggered global volcanism, a new study says.

Structures of Dwarf Satellites of Milky Way-like Galaxies: Morphology, Scaling Relations, and Intrinsic Shapes

The structure of a dwarf galaxy is an important probe of the effects of stellar feedback and environment. Using an unprecedented sample of 223 low-mass satellites from the ongoing Exploration of Local Volume Satellites survey, we explore the structures of dwarf satellites in the mass range 105.5 < M ⋆ < 108.5 M ⊙. We survey satellites around 80% of the massive, M K < − 22.4 mag, hosts in the Local Volume (LV). Our sample of dwarf satellites is complete to luminosities of M V <−9 mag and surface brightness μ 0,V < 26.5 mag arcsec−2 within at least ∼200 projected kpc of the hosts. For this sample, we find a median satellite luminosity of M V = −12.4 mag, median size of r e = 560 pc, median ellipticity of ϵ = 0.30, and median Sérsic index of n = 0.72. We separate the satellites into late- and early-type (29.6% and 70.4%, respectively). The mass–size relations are very similar between them within ∼5%, which indicates that the quenching and transformation of a late-type dwarf into an early-type one involves only very mild size evolution. Considering the distribution of apparent ellipticities, we infer the intrinsic shapes of the early- and late-type samples. Combining with literature samples, we find that both types of dwarfs are described roughly as oblate spheroids that get more spherical at fainter luminosities, but early-types are always rounder at fixed luminosity. Finally, we compare the LV satellites with dwarf samples from the cores of the Virgo and Fornax clusters. We find that the cluster satellites show similar scaling relations to the LV early-type dwarfs but are roughly 10% larger at fixed mass, which we interpret as being due to tidal heating in the cluster environments. The dwarf structure results presented here are a useful reference for simulations of dwarf galaxy formation and the transformation of dwarf irregulars into spheroidals.

Habitability of the early Earth: liquid water under a faint young Sun facilitated by strong tidal heating due to a closer Moon

Geological evidence suggests liquid water near the Earth’s surface as early as 4.4 gigayears ago when the faint young Sun only radiated about 70% of its modern power output. At this point, the Earth should have been a global snowball if it possessed atmospheric properties similar to those of the modern Earth. An extreme atmospheric greenhouse effect, an initially more massive Sun, release of heat acquired during the accretion process of protoplanetary material, and radioactivity of the early Earth material have been proposed as reservoirs or traps for heat. For now, the faint-young-Sun paradox persists as an important problem in our understanding of the origin of life on Earth. Here, we use the constant-phase-lag tidal theory to explore the possibility that the new-born Moon, which formed about 69 million years (Myr) after the ignition of the Sun, generated extreme tidal friction—and therefore, heat—in the Hadean and possibly the Archean Earth. We show that the Earth–Moon system has lost $${\sim }3~{\times }~10^{31}$$ ∼ 3 × 10 31 J (99% of its initial mechanical energy budget) as tidal heat. Tidal heating of $${\sim }10\,\mathrm{W\,m}^{-2}$$ ∼ 10 W m - 2 through the surface on a time scale of 100 Myr could have accounted for a temperature increase of up to $$5\,^\circ $$ 5 ∘ C on the early Earth. This heating effect alone does not solve the faint-young-Sun paradox but it could have played a key role in combination with other effects. Future studies of the interplay of tidal heating, the evolution of the solar power output, and the atmospheric (greenhouse) effects on the early Earth could help in solving the faint-young-Sun paradox.

The Lunar Fossil Figure in a Cassini State

The present ellipsoidal figure of the Moon requires a deformation that is significantly larger than the hydrostatic deformation in response to the present rotational and tidal potentials. This has long been explained as due to a fossil rotational and tidal deformation from a time when the Moon was closer to Earth. Previous studies constraining the orbital parameters at the time the fossil deformation was established find that high orbit eccentricities (e ≳ 0.2) are required at this ancient time, which is difficult to reconcile with the freezing of a fossil figure owing to the expected large tidal heating. We extend previous fossil deformation studies in several ways. First, we consider the effect of removing South Pole−Aitken (SPA) contributions from the present observed deformation using a nonaxially symmetric SPA model. Second, we use the assumption of an equilibrium Cassini state as an additional constraint, which allows us to consider the fossil deformation due to nonzero obliquity self-consistently. A fossil deformation established during Cassini state 1, 2, or 4 is consistent with the SPA-corrected present deformation. However, a fossil deformation established during Cassini state 2 or 4 requires large obliquity and orbit eccentricity (ϵ ∼ 68° and e ∼ 0.65), which are difficult to reconcile with the corresponding strong tidal heating. The most likely explanation is a fossil deformation established during Cassini state 1, with a small obliquity (ϵ ∼ −0.2°) and an orbit eccentricity range that includes zero eccentricity (0 ≤ e ≲ 0.3).

Effects of tidal heating in Proxima Centauri b's thermal evolution

&lt;p&gt;The recent discovery of a terrestrial planet orbiting Proxima Centauri, our closest neighbor (an M5.5V star of 0.1 M&lt;sub&gt;Sun&lt;/sub&gt; mass and only 1.3 pc distance to the Sun), offers an excellent planet laboratory to study the most important theories of planet evolution and composition. The planet (Proxima b) is orbiting the star in its habitable zone at a separation of only ~0.05 AU and an orbital period of ~11 days, and most recent studies suggest a non-zero eccentricity of about 0.17. With a mass of &gt;=1.2 M&lt;sub&gt;Earth&lt;/sub&gt;, Proxima b is expected to have a rocky composition, which might resemble the Earth. It is therefore an excellent target to characterize terrestrial planets similar to Earth, avoiding the inherent biases of only studying the terrestrial planets of the solar system.&lt;/p&gt; &lt;p&gt;Due to its close orbit and expected eccentricity, Proxima b most likely suffers from severe tidal heating, which can have an extreme incidence in the planet's habitability and the survival of an atmosphere. In this work, we perform a comprehensive analysis of the incidence that different distribution patterns of tidal heating can have on Proxima b's interior and thermal evolution. To accomplish this goal, we consider various possible geometries of the planet, from the simplest case, homogeneous distribution of the generated heat, to the more complicated cases, with an inhomogeneous distribution pattern that depends on the planet's interior structure (a stratified sphere, an incompressible homogeneous planet, or a two-layer structure with a differentiated core). The different models considered alter how tidal heat is distributed throughout the planet's interior, which can highly affect its overall thermal evolution.&lt;/p&gt; &lt;p&gt;Furthermore, due to its proximity to the central star, Proxima b may as well be in synchronous rotation with Proxima Centauri. This can cause an extreme surface temperature variation between the hemisphere that permanently faces the star and the opposite hemisphere. In this work, the effect that synchronous rotation may have on Proxima b's interior is also thoroughly investigated.&lt;/p&gt;